1. Need for Pinch-Point Protection
Operators of resistance (spot) welding machines are often exposed to potentially serious injury due to the pinch point area between the welding electrodes. A typical resistance welder operates with electrode forces of 250 lbs to 5,000 lbs and higher. Because the force is concentrated on a very small contact surface of the electrode, the force density is extremely high and can easily cause permanent finger injury or total amputation.
For example, on a welder with 600 lbs of electrode force and a ¼″ contact surface, the force density at the pinch point is 12,229 lb/in2. Because a common application of a resistance welding machine requires the operator to load parts between the electrodes as well as keep their hands on these parts during the welding sequence, the possibility for serious injury is present on a daily basis. This invention eliminates serious pinch-point injury by reducing the force between electrodes to a very low level when the operator's finger or other body part is between the electrodes.
2. Prior Art
A typical prior art arrangement will be described with reference to FIGS. 1 to 4. In one form of the prior art arrangement used on direct action welders (“press welders”) and shown in FIG. 1, a limit switch 1 is mounted on a fixed portion of the resistance-welding machine frame 2 and wired to the welder's control circuitry 3. An adjustable finger 4 is mounted on the moving ram 5 and is adjusted so that the limit switch closes only after the ram has lowered to the point where space between the electrodes 6 is below the desired dimension (typically about ¼″). After the initiation switch has been closed, the welder's ram 5 falls by gravity, or has some other mechanism to restrict force between the electrodes until the limit switch 1 has closed. The control circuitry 3, then turns another output on to place full welding force between the electrodes 6.
A second form of the prior art arrangement is used on rocker arm type welders and shown in FIG. 2. Force at electrodes 6 is derived from the force out of the rear piston multiplied by the mechanical advantage of arms C/B. A limit switch 7 for this prior art device is mounted on a fixed portion of the welder frame 2, and an adjustable finger 8 is installed on some portion of the welder's moving cylinder mechanism 9. This finger 8 is adjusted so that the limit switch 7 closes only after the electrodes 6 have closed to the point where space between them is below the desired dimension (typically about ¼″). The control then turns another output on to place full welding force between the electrodes 6.
However in both of the above prior arrangements, setting of the limit switch finger 4 or 8 is done by the operator or setup person, and the accuracy of this setting is fully in the hands of this person's skill. Furthermore, if the electrodes 6 are moved during normal production or subsequent electrode replacement or adjustment, and the limit switch is not adjusted properly, the safety of the system is compromised.
A third form of the prior art arrangement, as shown in FIG. 3, utilizes a system actuated by a motor 9 to close a limit switch 11 (or limit switch cam). During a setup sequence, an air cylinder is activated to bring the electrodes 6 together. Then the motor 9 moves the switch 11 or cam until the switch is closed by the cam, and then backs it up until a specific dimension (typically about ¼″) has been reached. During each welding sequence, electrodes 6 are moved together under reduced force until the limit switch 11 has been closed. The control then turns another output on to place full welding force between the electrodes 6.
This prior art allows automatic setting of the correct spacing between electrodes 6. However, in this third prior art arrangement, proper setting of the cam is mechanical and subject to mechanical adjustment errors. Additionally, as with the first two forms, if the electrodes 6 spacing is changed during normal production or subsequent electrode replacement or adjustment, and the operator does not remember to reset the finger the safety of the system is compromised.
A fourth form of the prior art arrangement, as shown in FIG. 4, utilizes a mechanically or pneumatically moved sensing arm (sometimes called “ring guard”) 11. When the welder is energized during each welding cycle, the electrodes 6 do not move forward, but this sensing arm 11, lowers to touch ring 11 a to the part 12 being welded. Sensing arm 11 is mechanically designed to encompass the area around the electrodes 6. If the distance traveled is past the set point on a limit switch 13, the sensing arm 11 will retract and the electrodes 6 will be closed under full welding force. If ring 11 a on the sensing arm 11 does not move this minimum set distance, as would happen when the operator's finger or other body part is under ring 11 a the sensing arm 11 will retract but the welder control will not cause the electrodes 6 to close.
However, in this fourth prior art arrangement if the welder operator or setup person does not adjust the sensing arm 11 properly, high force can be applied between the welding electrodes 6 even though the operator's finger or other body part is between the electrodes. Further, just bending this sensing arm 11 out of the proper sensing zone renders this system totally useless.